Twenty people and two fishing companies, based in Kilkeel, Northern Ireland, admitted flouting European laws designed to protect dwindling stocks of fish, such as cod and haddock.
The fishermen under-recorded the amount of protected "quota" fish by falsely claiming they were catching "non-quota" fish such as conger eel and gurnard.
The court heard the fishermen, who ran 12 boats in the Irish sea, caught fish worth more than £1.2 million (€1.82 million) in 2003 but only declared about £844,000 (€1.28 million) of that haul.
Liverpool Crown Court heard defence claims that the fishermen had not been motivated by greed but "commercial necessity".
Severe financial hardship caused by European fishing quotas forced fishermen to cheat the system in a practice known as "black fishing", the court was told
However, Judge Nigel Gilmour fined 17 of the defendants a total of £147,200 (€223,000) and ordered them to pay £110,500 (€168,000) in costs.
The remaining defendants - Kilkeel Fishselling Company, McBride Fishing Company, Charles Hubert McBride, Charles Leslie McBride and Leslie Girvan - will be sentenced at a later date following an investigation into their assets under the Proceeds of Crime Act.
Judge Gilmour said there had been a "cynical and deliberate disregard" for the regulations.
He said: "In 2003, the defendants were facing significant reductions in profits due to the quota system then prevailing. This financial difficulty, which was not of the defendants' own making, motivated these offences. It does not excuse the offences."
Michael Parker, district inspector for Britain's Marine Fisheries Agency (MFA), said: "The MFA is charged to enforce robustly the rules professionally, consistently and fairly. We are determined to seek out those who fish illegally and bring them before the courts - not least because those who break the law are damaging the chances of other fishermen. One key job of the MFA is collecting data on fish caught and landed so that Brussels can decide on fish quotas - the amount of fish of different species that can be landed without damaging stocks. That task is made more difficult the more cheating that goes on."